This invention relates to ophthalmic analytical and diagnostic systems, and in particular the invention is concerned with obtaining accurate determinations of the shape of the human eye structures such as the cornea, the lens and the retina. An apparatus in accordance with the invention measures, calculates and displays the shape of selected cross sections of the cornea, for example, and is intended for use by ophthalmic surgeons as well as the eye care community at large.
One of the principal activities of the eye care specialist, which includes both ophthalmologists and optometrists, is to determine the refractive power of the eye as an optical system. Since the only major refractive index change along a light path entering the eye to impinge on the retina occurs on the first air to tear layer interface or, approximately, at the corneal anterior epithelial surface, the precise measurement of the shape of the corneal epithelial surface is the key to estimating the refractive power of a given eye.
Traditionally, the eye care specialist has been satisfied with a measurement from keratometric readings. The keratometric readings ("K-readings") correspond to the curvature of the corneal epithelial surface at the intersection of the corneal epithelial surface with the central visual axis of the eye. The K-readings are usually displayed in diopter power which is proportional to the reciprocal of the radius of curvature. The K-readings provided by keratometers correspond to the curvatures at one point on the corneal epithelial surfaces along two surface rays passing through that point. Usually, the two rays match the semi-major and semi-minor axes of the eye which are the nasal-temporal (horizontal) axis and the superior-inferior (vertical) axis of the eye.
Since the first concern of the eye care specialist is central, axial vision, the K-readings, which only provide two curvature measurements along the semi-major and semi-minor axis normal to the visual axis, represent a fair estimate of refractive power along the most critical light paths in the human eye.
To date, the eye care community has relied on the eye surface being approximated as a combination of a sphere and a cylinder, thus the reference to 20/20 as a visual standard. This approximation is exact at the intersection of the corneal anterior surface with the visual axis. The approximation is known to fail as one proceeds radially outward from the central visual axis towards the limbus, roughly the outermost edge of the cornea where the triple-point transitions between cornea, sclera, and iris tissues take place.
There are several instruments for measuring the location of the corneal anterior surface in proximity of the limbus as well as in the central region, but the display generated from these measurements usually assumes that the eye can also be approximated as a combination of spheres and cylinders. In a sense, these instruments spread the error in approximating the shape of the corneal surface from being concentrated toward the limbus to being distributed over a greater region.
Notable exceptions are instruments based on confocal microscopy that measure the actual curvatures without simplifying assumptions. However, systems based on confocal microscopy have very limited fields of view, considerably smaller than the full corneal surface. Such systems must then rely on a sequence of measurements over time which are subsequently made to piece together using either fractal techniques or some boundary matching algorithm. These paste-ups involve some form of interpolation, albeit on the boundary of the images rather than in the interior. In contrast with interpolations which are based on a sparse set of measurements, confocal techniques provide dense measurements at the expense of not having them performed simultaneously. Even though the sequential measurements can be formed very quickly, involuntary eye motion is known to occur within millisecond time scales, faster than the time required to complete data gathering using confocal techniques. This can introduce errors.
Since only a finite number of measurements of the actual location of the corneal anterior surface are possible, interpolation techniques are an intrinsic part of displaying a continuous shape based on the measured information. In most instruments still in use by the eye care community, the error in measuring the shape of the corneal anterior surface is often not in the measurement technique but in the numerical interpolation techniques utilized to prepare the display of the continuous corneal surface cross-section.